It could be ether hardware or software problem. try installing the scanner on a friends computer and see if it does the same thing. if it does then your scanner if faulty, if not then try uninstalling you scanner drivers restart your computer and go to http://www.mustek.com.tw/Download/driv_scanr/europe/b4800tapro.html to download the new drivers to reinstall.
Answers & Comments
It could be ether hardware or software problem. try installing the scanner on a friends computer and see if it does the same thing. if it does then your scanner if faulty, if not then try uninstalling you scanner drivers restart your computer and go to http://www.mustek.com.tw/Download/driv_scanr/europe/b4800tapro.html
to download the new drivers to reinstall.